L-Asparaginase has been used in the treatment of acute lymphoblastic leukemia, leukemic lymphosarcoma and lymphosarcoma. Also, it may be of some value as an immunosuppressive agent in treating organ transplant rejection and autoimmune diseases. However, the enzyme from microbial sources has a short biological half-life, is also immunogenic and exerts some toxicity. To extend the biological half-life and to modify the immunogenic and toxic properties of L-asparaginase we propose to immobilize the enzyme in vivo. Derivatives of L-asparaginase designed to react with tissue functional groups such as sulfhydryl groups will be prepared. Mice will be given a single injection of each enzyme derivative and serum levels of L-aspargine will be compared to those in animals that received a single injection of unmodified L- asparaginase. Various routes of administration (I.V., I.P., I.M.) will be utilized to compare the enzyme derivatives with the native enzyme. In addition, mice will be inoculated with lymphosarcoma 6C3HED and subsequently treated with a single injection of reactive enzyme derivative or native enzyme to compare the efficacy of treatment. The immunogenicity of the L-asparaginase derivatives will be compared to that of the unmodified enzyme in rabbits. Successful development of the above concepts might lead to other therapeutic systems involving enzymes. For example, lipoprotein lipase and lecithin cholesterol acyl transferase might be immobilized in the vascular system to help clear serum lipoproteins. The immunogenicity of immobilized macromolecules can be exploited in the immunotherapy of cancer. Mice bearing solid lymphosarcoma 6C3HED tumors will be immunized against a potent antigen. The antigen will be immobilized in the tumor by intratumor or vascular administration of a reactive derivative of the antigen. An immune response directed at the immobilized antigen might destroy the tumor cells. Alternatively, the antigen could be immobilized in the tumor to develop an immune reaction without prior immunization with the antigen. Other macromolecules can be immobilized in tumors to exert a cytotoxid effect. Solid lymphosarcoma 6C3HED tumors in mice will be treated with reactive derivatives of diphtheria toxin or enzymes that yield toxic products, such as lipid hydroperoxides from lipoxygenase activity. Immobilization of these toxic macromolecules in the tumor may result in selective cytotoxicity.